a) Field of the Invention
This intention relates to a new or improved process for producing smooth surfaced oriented strand board (OSB) panels. It also relates to a new or improved process for producing a thin and uniform mat of high density made of bonded wood particles, fibres or strands. The thin mat is produced with an uncured adhesive on one of its surfaces and a completely cured adhesive on the other surface. The thin mat could thus be bonded to another mat or to any other substrate (e.g. wood door core)in a one step pressing operation. The end product has a smooth and uniform surface, which could be painted or laminated. This invention, although more documented for OSB, is not limited to OSB panels as it could be employed to improve the surface quality of any type of panel or substrate. This approach could be employed for example to obtain a higher surface density (higher bending properties) with MDF or particleboard panels. Better surface quality will allow improved mechanical properties or allow a reduction of expensive adhesive needed to obtain panels of high mechanical properties.
b) Description of the Prior Art
OSB panels have found widespread application in the building industry, e.g. for use as sheathing and flooring in residential and other structures. OSB panels have numerous advantages compared with other wood based panels. For example, they are lower in cost and provide greater versatility in size than plywood panels.
As compared to particleboard panels, OSB panels possess many desirable properties, such as: high bending strength; high rigidity; and low linear expansion. Since OSB is bonded with phenolic or isocyanate resin, it is resistant to water hydrolysis and it does not have any significant formaldehyde emission problems.
Apart from its traditional uses in the construction of walls, roofs, floors and as a sheathing material, OSB could potentially also be employed as a substrate for lamination, at least in specialty applications where its superior mechanical properties are required.
It has been recognized that an OSB panel having a smooth surface or overlay could be sold at a substantially higher price in comparison to a regular OSB panel, and numerous efforts have been made to produce such a smooth surfaced panel and thus expand the market for OSB panels. Various commercial products have appeared in which OSB panels are coated e.g. with an overlay of fine particles or of fibre, but none has been entirely successful. One method (U.S. Pat. No. 5,425,976; 1995) consisted of the utilization of wood strands of higher density (450 kg/m.sup.3 to 600 kg/m.sup.3) which are difficult to compress. During pressing, the stiff strands in the OSB baseboard would force the overlaying fibre to take up most of the compaction. This method may reduce the areas of low density at the panel surface but fail to eliminate the problem. Low density areas at the panel surface are still present. Furthermore, since trembling aspen employed in most Canadian OSB mills has an oven-dry density of approximately 424 kg/m.sup.3, this process would have limited use. A multilayer panel using a gradation of OSB strands, slivers and fines has also been described to help prevent telegraphing and reducing sieving of the fines through the mat (U.S. Pat. No. 4,364,984; 1982). This method may reduce the number of sites with low density areas, but these are not eliminated. Areas of low density are still present, particularly in the panel top surface of panels due to sieving of the small fines particles through the mat which occurs especially when the mat is transported to the press on a moving line in a mill operation. Pre-pressing at ambient temperature has also often been suggested to densify the fines or fibres and improve surface quality (U.S. Pat. No. 2,343,740; 1944; U.S. Pat. No. 3,308,013; 1967; U.S. Pat. No. 4,883,456; 1989). Cold pre-pressing however provides limited compaction and requires high tack adhesives which are generally avoided in OSB mills due to potential build up in the blender and processing line.
In the course of this work, a technique for pre-pressing resinated fines between hot and cold platens was developed. The resulting thin mat was cured on the outside face, but not on the inside, so that it could be laminated onto an OSB mat prior to pressing. The process is quite forgiving and produces high density uniform and smooth surfaces, suitable for painting or lamination using less fines. Improved "saw cut" edges were also obtained using this approach.